Method and apparatus for implementing frequency-hopping in a base station

ABSTRACT

For implementing frequency-hopping in a base station, of a digital cellular radio system a first signal modulated by a base band data signal and transmitted on a first carrier frequency. The signal is divided in the time domain into several time slots, of which one time slot transmits the common control channel of the base station equipment and the other time slots transmit information regarding different connections. For an economical implementation of frequency-hopping in a base station having one transceiver unit, the base station further transmits a second signal modulated by the same base band data signal, on a second carrier frequency. The second signal is divided in the time domain into several time slots, each of which transmits the same information as the first signal, except the time slot that transmits the common control channel of the base station equipment. The second carrier frequency (27) is changed time-slot-specifically so that its frequency is the same as, or different from, the first carrier frequency.

This application claims benefit of international applicationPCT/FI95/00142 filed Mar. 16, 1995.

BACKGROUND OF THE INVENTION

The present invention relates to a method for implementingfrequency-hopping in a base station of a digital cellular radio system.In the base station, a first signal modulated by base band data signalis transmitted on a certain carrier frequency. The signal is divided inthe time domain into several time slots of which one time slot transmitsthe common control channel of the base station equipment and the othertime slots transmit information regarding different connections.

In cellular radio systems, the quality of a radio channel varies as afunction of time and place. On many occasions, a transmitting antennaand a receiving antenna are not within sight of each other due toobstacles formed by terrain or buildings in the line of sight, and thusthe signal detected at the receiver is a sum of beams that havetravelled different paths and have been reflected from obstacles andthat are, further, with different phases. The sum of several signalswith different phases follows the Rayleigh distribution in cases ofrandomly distributed phases.

Signal fading is, furthermore, dependent on the frequency of the signal.Thus, if a frequency difference between two signals is great enough,their fadings do not correlate. A difference of 1 MHz, for example, islarge enough for signal fadings to be independent of each other.

The frequency selective fading of a signal described above is one reasonfor the introduction of frequency-hopping technology in digital cellularradio networks. Frequency-hopping means that the transmission frequencyused in a connection is changed at predetermined intervals. Due tofrequency-hopping, the transmission quality can be improved especiallyin situations in which the terminal equipment moves very slowly, or notat all, as is often the case with, for example, hand-held phones.

Apart from the frequency diversity achieved by frequency-hopping, themethod is also advantageous in distributing the interferece caused bythe radio connection onto several frequencies, in which case momentaryinterference on any one frequency will remain small.

In the prior art solutions, if frequency-hopping has been desired in acell served by a base station, the base station equipment must have beenprovided with at least two complete transceiver units which contain baseband sections and radio frequency sections. Each unit, as such can forman independent base station, but to implement frequency-hopping severalcomplete transceiver units have been required in the same cell. This hasresulted in that the base station equipment capable of frequency-hoppinghas been disproportionately large in small cells in comparison with thecapacity needed, and the implementation of frequency-hopping has thusrequired heavy investments.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to implementfrequency-hopping in a base station equipment economically and withoutcreating excessive capacity.

This is achieved with the method of the type set forth in theintroduction, which is characterized in that the base station furthertransmits a second signal modulated by the same base band data signal ona certain carrier frequency, which second signal is divided in the timedomain into several time slots, each of which transmits the sameinformation as the first signal, except the time slot which transmitsthe common control channel of the base band equipment, and which carrierfrequency is changed time-slot-specifically so that the frequency iseither the same, as, or different from, the carrier frequency.

The invention also relates to a base station equipment in a digitalcellular radio system, this equipment comprising means for generating abase band data signal, a first means for transmitting a signal modulatedby a certain carrier frequency, and this signal being divided in thetime domain into several time slots of which one transmits the commoncontrol channel (CCCH) of the base station equipment and the otherswhich transmit information regarding different connections, and anantenna, characterized in that the equipment further comprises a secondmeans for transmitting a signal modulated by a carrier frequency,elements for changing the carrier frequency used in the second meanstime-slot-specifically, and means for distributing the base band signalto the first and the second transmitting means so that the signaltransmitted by the second means is divided in the time domain intoseveral time slots of which each transmits the same information as thesignal transmitted by the first means except the time slot thattransmits the common control channel (CCCH) of the base stationequipment.

The method of the invention can be applied for implementingfrequency-hopping in a cell comprising one transceiver unit. The basestation in accordance with the invention can be built similarly to aconventional frequency non-hopping base station, the only necessarychanges being in the RF section of the transmitter. Only one base bandsection is needed. If frequency-hopping is unnecessary, the base stationof the invention can easily implement space diversity in a directionfrom the base station to the mobile station. The use of frequencies canbe optimized so that frequency-hopping is utilized only on weakconnections.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail withreference to the examples in accordance with the accompanying drawings,in which:

FIG. 1 illustrates time-slot-specific frequency-hopping,

FIG. 2 shows an example of the structure of a base station equipment inaccordance with the invention,

FIG. 3 shows another example of the structure of a base stationequipment in accordance with the invention, and

FIGS. 4a-4b describe the time slot construction of the output signal ofthe transmitting means.

DETAILED DESCRIPTION

The invention can thus be applied in the base station equipment ofdigital cellular radio systems such as the GSM system, for instance. Inthe following, the invention will be described when applied in a timeslot structure of the type used in the GSM system, but without beingrestricted to that.

FIG. 1 illustrates the implementation of time-slot-specificfrequency-hopping. The horizontal axis of the figure represents time,and the vertical axis represents frequency. The figure shows thefrequency changes of one channel according to time slots. The basestation changes carrier frequency at the beginning of every time slot,but the frequency is not changed during the time slot. The frequency ischanged according to a predetermined sequence, in which case thereceiver can monitor the hopping and receive the information containedin each time slot on a correct frequency. In a base station, eachconnection can have a hop sequence of its own, in which case thefrequencies used in the time slots do not overlap.

FIG. 2 is a block diagram of the structure of a base station equipmentin which the method of the invention is applied. The equipment containsmeans 15 for generating a base band signal. The output 16 of the baseband signal-generating means 15 is connected to two transmitter units10,18 with the aid of a base band signal divider. The first transmitterunit 10 contains a carrier generator whose output 26 is connected to amodulating means 13 whose second input is a base band data signal fromthe base band signal divider 17. The time division carrier signal 23modulated by a base band signal is fed via an output amplifier 14 to anantenna 11. The first signal of the transmitter unit contains the commoncontrol channel CCCH of a cell served by a base station. By listening tothat signal, mobile stations obtain information of the base station sothat they can contact it, if necessary. In a GSM system, traffic on theCCCH channel is transmitted in the first time slot of a frame, indicatedby the number 0. The other time slots, of which there are seven in a GSMsystem, transmit a signal that is either data belonging to a connectionor, if the time slot is not in use, is a dummy burst. FIG. 4aillustrates the structure of a GSM frame in which the first time slot 0contains the CCCH channel, the second time slot 1 a dummy burst, timeslots 2, 3, 4 and 6 data information, and time slots 5 and 7 each adummy burst. The carrier frequency of the transmitter 10 is marked withf1, and it is constant.

The second transmitter unit 18 correspondingly contains acarrier-generator 20 whose output 27 is connected to modulation means 21whose second input is the base band data signal from the base bandsignal divider 17, i.e. the same signal as in the input of the firsttransmitter unit. In the example of the figure, the time divisioncarrier signal 24 modulated by the base band signal is supplied viaoutput amplifier 22 to an antenna 19. The signal of the secondtransmitter unit does not contain the common control channel CCCH, andthe time slot is not transmitted. In the other time slots 1-7, databelonging to some connection is transmitted, or the transmitter isswitched off, if the time slot is not in use.

The carrier frequency used by the second transmitter unit hopstime-slot-specifically under the control of the means 25, so that witheach connection (i.e. time slot) the frequency changes according to apredetermined sequence. In a base station in accordance with FIG. 2, inwhich two antennas are in use, the carrier frequency of the secondtransmitter unit can momentarily be the same as the frequency f1 used bythe first transmitter unit.

According to a second embodiment, when both of the transmitter units areon the same frequency, the second transmitter unit is switched off toavoid simultaneous transmission.

The second transmitter unit can be controlled time-slot-specifically sothat frequency-hopping is only performed in certain time slots. If twoantennas are in use in connections not applying frequency-hopping, bothtransmitters transmit the same signal on the same frequency, and spacediversity is obtained.

Frequency use can be optimized so that frequency-hopping is only appliedto poor quality connections. To judge communication quality, signalstrength as well as bit error and signal-to-noise ratios can be used asstandards.

FIG. 3 is a block diagram representation of an alternative structure ofa base station equipment for implementing the method of the invention.The equipment contains means 30 for combining signals from twotransmitter units 10, 18 for transmitting via one antenna 11. Theoperation of a base station such as the one in FIG. 3 is similar to thedescribed above, except that when one antenna is used, the secondtransmitter unit is switched off in cases where its carrier frequency isthe same as the frequency of the first transmitter unit.

In addition to the components described above, the exemplary basestations also contain components other than those described, such asfilters and divider amplifiers, but due to reasons of clarity they havebeen omitted from the figure, as they are not essential to understandingthe present invention.

As explained above, FIG. 4a illustrates a frame structure transmitted bythe first transmitter unit, the first time slot 0 containing the CCCHchannel, time slots 2, 3, 4 and 6 containing data information (C1-C4),and time slots 1, 5 and 7 containing a dummy burst (B1-B3). The carrierfrequency of the first transmitter is indicated by f1, and it isconstant.

FIG. 4b illustrates a frame structure transmitted by the secondtransmitter unit. In the example illustrated in the figure, thetransmitter unit utilizes three different frequencies, f2, f3 and f4 infrequency-hopping. The number of frequencies can, of course, besomething else in practice. Let us assume that connections C1, C2 and C4using time slots 2, 3 and 6 employ frequency-hopping, but Connection C3using time slot 4 does not hop. In such a case the second transmitterunit transmits a signal in time slots 2, 3 and 6 as shown by the figure,using at each moment of time one of the carrier frequencies f2-f4. Thesecond transmitter unit can, depending on the application, also use thefrequency f1 of the first transmitter unit. The frequency used at eachconnection and each moment of time depends on the chosen hoppingsequence which is specific for each connection.

The second transmitter unit can, for example, transmit a frame inaccordance with FIG. 4b so that time slot 2 uses frequency f2, time slot3 frequency f3 and time slot 6 frequency f4. In the next frame, the timeslots are changed so that time slot 2 uses frequency f3, time slot 3frequency f4 and time slot 6 frequency f3. The frequencies of the timeslots are independent of each other, and they can also be the same.

When two separate antennas are used, the second transmitter unit is ableto transmit on non-hopping connections the time slot in question on thesame frequency as the first transmitter unit, in which case spacediversity is achieved at the transmitting end. In the example describedabove, the second transmitter unit can thus transmit the signal ofconnection C4 in time slot 6 on a frequency f1.

Even when using two separate antennas the second transmitter unit can beswitched off if the frequencies of the transmitter units are the same;this can de done to avoid simultaneous transmission.

Even though the invention has been described in the above with referenceto the examples in accordance with the accompanying drawings, it isobvious that the invention is not restricted to them but can be modifiedin a variety of ways within the scope of the inventive idea disclosed inthe attached claims.

I claim:
 1. A base station equipment in a digital cellular radio system,said equipment comprising:means for generating a base band data signal;a first means for transmitting a signal modulated by a first carrierfrequency, said signal being divided in the time domain into severaltime slots of which one transmits a common control channel of the basestation equipment and the other time slots transmit informationregarding different connections; an antenna; a second means fortransmitting a signal modulated by a second carrier frequency; elementsfor changing said second carrier frequency time-slot-specifically; andmeans for distributing said base band signal to the first and the secondtransmitting means so that the signal transmitted by said second meansis divided in the time domain into several time slots of which eachtransmits the same information as the signal transmitted by said firstmeans, except said time slot that transmits the common control channelof the base station equipment.
 2. The equipment as claimed in claim 1,wherein said first and the second means each comprise:means forgenerating a carrier wave; means for modulating the carrier wave by abase band signal; and means for amplifying the modulated signal.
 3. Theequipment as claimed in claim 1, further comprising:an amplifier means;and a second antenna which is connected to said amplifier means.
 4. Theequipment as claimed in claim 1 further comprising:means for combiningsaid signals from said first and second transmitting means, fortransmission via said antenna.
 5. The equipment as claimed in claim 1wherein:said elements for changing the second carrier frequency arearranged to change the second carrier frequency according to apredetermined sequence.
 6. The equipment as claimed in claim 5, forchanging the second carrier frequency are arranged to switch off saidsecond means when the frequency indicated by said predetermined sequenceis the same as the frequency used by said first means.
 7. A method forimplementing frequency-hopping in a base station equipment of a digitalcellular radio system, comprising:transmitting by said base station afirst signal modulated by a base band data signal, on a first carrierfrequency, which signal is divided in the time domain into several timeslots of which one time slot transmits a common control channel of thebase station equipment and the other time slots transmit informationregarding different connections; further transmitting by said basestation a second signal modulated by the same base band data signal, ona second carrier frequency, which second signal is divided in the timedomain into several time slots each of which transmits the sameinformation as said first signal, except said time slot which transmitsthe common control channel of the base station equipment, and whichsecond carrier frequency is changed time-slot-specifically so that thefrequency thereof is either the same as, or different from, said firstcarrier frequency.
 8. The method as claimed in claim 7, wherein:saidsecond carrier frequency is changed time-slot-specifically according toa predetermined sequence.
 9. The method as claimed in claim 7,wherein:both said first and the second signals are transmitted viarespective antennas.
 10. The method as claimed in claim 7, wherein:bothsaid first and the second signals are transmitted via a same antenna.11. The method as claimed in claim 8, wherein:if said second carrierfrequency, as indicated by said predetermined sequence to be used bysaid second signal is the same in each time slot as said first carrierfrequency to be used by said first signal, said second signal is nottransmitted.
 12. The method as claimed in claim 7, wherein;said secondsignal is transmitted time-slot-specifically when signal quality ofsignaling send from said base station in a respective time slot, asdetected at a receiving end, is poorer than a predetermined thresholdvalue.